Displays, photovoltaics, microelectronics, and MEMS (microelectro-mechanical devices) employ polycrystalline silicon (poly-Si) films. There is a need to be able to fabricate such poly-Si films at low temperatures, using low-cost processing techniques. Poly-Si is used in thin film transistors (TFTs) for the active channel region because of its relatively high mobility and in solar cells for the same reason; i.e., its high carrier mobility.
Low resistance metal/poly-Si silicide materials are also needed for the source and drain contact or contact finger regions of TFTs and for the contact regions of solar cells. Such silicides offer low resistance, chemical stability, and relatively low temperatures of formation. Low resistance metal silicides, formed with poly-Si also are useful as resistors and interconnects in displays, microelectronics, and MEMS, if the metal/poly-Si silicide regions can be readily isolated. Such metal silicide regions can also be subsequently used to initiate electrochemical or electroless deposition of additional metallic layers. In such case, the metal/poly-Si silicide can serve as a foundation for subsequent depositions for further tailoring of resistance.
Metal silicides are currently fabricated by reacting co-deposited or sequentially deposited silicon and metal or by a deposited metal film with a silicon substrate or film. This is often done by vacuum depositing the metal, patterning it lithographically, and then reacting the metal with the silicon substrate, using an elevated temperature process.
Thin film poly-Si films can be directly deposited or deposited in amorphous silicon (a-Si) form and then crystallized into poly-Si films. In the latter approach, crystallization of the a-Si into poly-Si can be achieved by various methods: laser crystallization, solid phase crystallization, and metal induced solid phase crystallization. The metal induced crystallization (MIC) SPC process has been also used to make poly-Si devices in a pre-determined pattern to cause selective crystallization with a low thermal budget. For poly-Si films produced by MICSPC, the metals (e.g., Pd, Ni, Cr or Mo) have been physically deposited in vacuum or from solution. See, for example, U.S. Pat. Nos. 5,147,826 and 5,275,851.
Regardless of the method used to fabricate poly-Si films, the resulting grain size of the poly-Si film plays a crucial role in determining the performance of a device. Grain size affects critical device parameters such as carrier mobilities in transistors, thin film transistors (TFTs), solar cells, detectors; on/off ratio of transistors and TFTs; switching speed of transistors and TFTs; and efficiencies of solar cells. However, none of the methods used to produce thin film poly-Si (including the as-deposited approach), has been able to produce grain sizes larger than on the order of tens of microns (e.g., 10.about.50 .mu.m), i.e., see Lee et al., Appl. Phys. Lett. 66, 1671 (1995).
There is a need for a low cost, low temperature method for inducing crystallization in amorphous semiconductors. Also there is a need to produce large crystals during such a crystallization procedure.